1. Field of the Invention
The present invention relates to a method for rust prevention and hydrophilization treatment of the surfaces of a flux-brazed aluminum-made heat exchanger, which method is capable of imparting the surfaces thereof with excellent deodorization effect in addition to favorable rust prevention and hydrophilization effect.
2. Description of the Related Arts
A heat exchanger to be used for car air conditioners is constructed in such a complicated manner that aluminum-made fins are held usually at small spacing in order to make the most of the surface area of the heat exchanger, and further aluminum tubes for supplying the fins with a cooling medium are intricately arranged. Moisture in the air which is attached to the surfaces of the fins as condensate during the operation of an air conditioner becomes water droplets of almost semispherical shape on the surfaces of the fins that are inferior in wettability, and the resultant condensate resides in the form of bridge among the fins, thereby preventing smooth flow of suction air and increasing draft resistance in the heat exchanger. It follows that the inferior wettability or water repellency of the surfaces of fins deteriorates the heat exchange efficiency.
In addition, although aluminum and an alloy thereof are inherently excellent in corrosion resistance, when condensate resides on the surfaces of the fins for a long time, oxygen concentration cells are formed or pollutants in the air are gradually attached thereto and concentrated, thus promoting hydration or corrosion reaction. The corrosion products deposit on the surfaces of the fins, impair heat exchange characteristics and besides, during heating operation in winter are made into white particulate and exhausted along with warm air through a fan.
In order to improve the above-mentioned problems, there are proposed various surface treatment agents and surface treatment methods for the purpose of imparting hydrophilicity, wettability, corrosion resistance, deodorization properties and the like to the surfaces of the aluminum fins. There are exemplified, for instance, the surface treatment agents composed of a mixture of hydrophilic resins such as polyvinyl alcohol, polyvinyl pyrrolidone, water-soluble nylon and a water-soluble phenolic resin (JP5-302042A); those composed of specific modified polyvinyl alcohol, a phosphorus compound salt and/or a boron compound salt and the like (JP2003-003282A).
On the other hand, an aluminum-made heat exchanger is assembled by joining a large number of aluminum fins and aluminum tubes as mentioned above, but the fins and tubes cannot be readily bonded by brazing, soldering and the like that are other than a mechanical bonding method, since the surfaces of aluminum are covered with strong dense oxide films, whereby as a brazing method, a VB method (vacuum brazing method) comprising brazing under vacuum has hitherto been put into practice.
However, a halogen base flux has recently been developed as means for effectively removing and destroying the oxide films, and thus a flux brazing typified by NB method (Nocolock method) comprising brazing in an atmosphere of nitrogen has been prevailingly employed by reason of easy management of brazing, inexpensive furnace, inexpensive brazing processing and the like factors.
Nevertheless the NB method involves a problem in that a flux, which remains on the surfaces of aluminum, results in non-uniform surface conditions, and makes it impossible to achieve uniform surface treatment by a chemical conversion treatment, a hydrophilization treatment and the like, thereby causing insufficient corrosion resistance, adhesiveness and the like.
Under such circumstances, in regard to the surface treatment of a flux-brazed aluminum-made heat exchanger, there are presently carried out in turn (1) flux removal step, (2) chemical conversion treatment step (rust prevention step) and (3) hydrophilization treatment step. However the treatment method just mentioned involves the problems of too many steps, and besides the generation of halogen-based wastewater in the flux removal step. Moreover, flux removal step, in which etching is conducted with an acid and/or an alkali to enhance chemical conversion treatment properties, also involves a problem in that the flux cannot be removed alone, thus making it impossible to achieve uniform chemical conversion treatment owing to the occurrence of excessive etching to expose silicon in a brazing material (Al/Si).
Furthermore, Cr6+ free chemical conversion treatment, which is made necessary from the safety problem of human bodies, is insufficient in corrosion resistance.